1. Field of the Invention
The present invention pertains to the art of washing machines and, more particularly, to an unbalance detection system for a washing machine.
2. Discussion of the Prior Art
During operation of a washing machine, it is not uncommon for a tub or spinner of the machine to become unbalanced due to the particular distribution of a load of clothes therein. When the tub is rotated at a relatively high speed during an extraction phase of an overall washing cycle, an unbalanced condition can cause considerable vibrations of the entire machine. Since excessive vibrations can be detrimental to the continued reliability of the machine, it is known in the art to provide a vibration detection system for sensing an actual or incipient unbalance condition and for altering the operation of the machine when a predetermined threshold is reached. Typically, known systems function to either reduce the rotational speed of the clothes tub or entirely shut down the machine to counteract an unbalance condition.
In the art, various different vibration detection systems have been employed. For instance, it has been known to employ switches, such as mercury or micro-switches, which are engaged when excessive vibrations are encountered. Activation of these switches is relayed to a controller for altering the operational state of the machine. Other known systems provide electronic sensing systems to perform a corresponding function. For example, an unbalance condition which develops during a spin cycle can be sensing by sampling speed variations. However, the actual speed fluctuations can be slight, such that the speed based control needs to be tight, i.e., extremely sensitive and accurately controlled. On the other hand, torque based systems are also known as exemplified by U.S. Pat. Nos. 2,917,175 and 4,765,161. In such systems, motor current is sensed and used as a parameter related to motor torque. However, given that the actual current value can fluctuate based on various factors and such systems typically require the inclusion of an additional sensor for sending current signals which are averaged over a certain period of time or number of basket revolutions, shortcomings can exist in at least the cost and precision of such systems.
In any event, there exists a need in the art for an unbalance detection system for a washing machine, particularly a horizontal axis washing machine, which can effectively, efficiently and accurately sense an unbalance condition. Furthermore, there exists a need for an improved unbalance detection system which is simple in construction and operation, so as to be reliable and cost effective.
A washing machine constructed in accordance with the present invention incorporates a system for controlling either an actual or incipient unbalance condition in a reliable, accurate and cost effective manner. More specifically, the present invention is directed to an unbalance detection system for a washing machine, particularly a horizontal axis washing machine, which can sense an unbalance condition through fluctuations in command control signals sent to a motor controller for a basket of the washing machine spinning at a plaster speed.
More particularly, the washing basket or tub of a tumble-type washing machine is rotated at a predetermined speed, i.e., in the order of 85 rpm in accordance with the most preferred form of the invention, during a portion of a spin cycle. During this portion of the spin cycle, clothes located in the washing tub are plastered against the annular inner wall surface of the tub wherein water is extracted from the clothes and directed to a drain. To achieve the desired spin speed, a main electronic controller provides a pulse width modulated (PWM) command signal to a drive motor controller for the washing tub. In return, the motor control sends feedback signals to the main controller regarding the rpm achieved for each revolution of the tub. The actual PWM signal is directly for each revolution of the tub. The actual PWM signal is directly proportional to the torque at the drive motor such that, if imbalances occur, torque variations will exist. That is, if an unbalanced load is to be lifted during rotation of a horizontal axis washing tub, a large torque will be needed. When the load drops, the torque will be considerably smaller. Fluctuations in the PWM command signal are sensed in accordance with the unbalance detection system of the present invention in order to reflect these torque variations. If a predetermined unbalance condition arises, spinning of the washing tub is terminated, the load in the tub is tumbled to redistribute the load and then the spin cycle is again attempted.
With this construction and operation, a relatively simple, inexpensive and effective unbalance detection system for a washing machine is provided. In any event, additional objects, features and advantages of the invention will become more readily apparent from the following detailed description of a preferred embodiment of the invention, when taken in conjunction with the drawings wherein like reference numerals refer to corresponding parts in the several views.